A screen for antibiotic resistance determinants reveals a fitness cost of the flagellum in Pseudomonas aeruginosa.

The intrinsic resistance of Pseudomonas aeruginosa to many antibiotics limits treatment options for pseudomonal infections. P. aeruginosa9s outer membrane is highly impermeable and decreases antibiotic entry into the cell. We used an unbiased, high-throughput approach to examine mechanisms underlying outer membrane-mediated antibiotic exclusion. Insertion Sequencing (INSeq) identified genes that altered fitness in the presence of linezolid, rifampicin, and vancomycin, antibiotics to which P. aeruginosa is intrinsically resistant. We reasoned that resistance to at least one of these antibiotics would depend on outer membrane barrier function, as previously demonstrated in Escherichia coli and Vibrio cholerae. This approach demonstrated a critical role of the outer membrane barrier in vancomycin fitness, while efflux pumps were primary contributors to fitness in the presence of linezolid and rifampicin. Disruption of flagellar assembly or function was sufficient to confer a fitness advantage to bacteria exposed to vancomycin. These findings clearly show that loss of flagellar function alone can confer a fitness advantage in the presence of an antibiotic. Importance The cell envelope of Gram-negative bacteria renders them intrinsically resistant to many classes of antibiotics. We used Insertion Sequencing to identify genes whose disruption altered the fitness of a highly antibiotic resistant pathogen, Pseudomonas aeruginosa, in the presence of antibiotics usually excluded by the cell envelope. This screen identified gene products involved in outer membrane biogenesis and homeostasis, respiration and efflux as important contributors to fitness. An unanticipated fitness cost of flagellar assembly and function in the presence of the glycopeptide antibiotic vancomycin was further characterized. These findings have clinical relevance for individuals with Cystic Fibrosis who are infected with P. aeruginosa and undergo treatment with vancomycin for a concurrent Staphylococcus aureus infection.